1. Field of the Invention
This invention relates to plasma generators which generate with radio-frequency (rf) energy a plasma for etching semiconductor wafers in the semiconductor fabrication industry. More particular, it pertains to improvements in the cap and diffusor used in forming an etching plasma between a hot energy-emitting electrode and a semiconductor wafer functioning as a ground electrode.
2. Prior Art
Plasma etching apparatus usable with treatment of semiconductor wafers are seen in U.S. Pat. Nos. 4,094,722; 4,245,154; 4,252,595; and 4,282,077. In each, an etching gas activated by an electrically-generated plasma is used to etch various surfaces of a wafer.
In a plasma generator wafer etching machine known as the SIGMA 80 machine, sold by Branson/IPC, a SmithKline-Beckman company of Hayward, Calif., and described in Solid State Technology, August 1982, pages 88-92, a plasma is formed across a hot rf emitting electrode and a ground electrode in the form of a semiconductor wafer work-piece. The wafer work-piece is held in a pivotable chuck in parallelism with the hot electrode. An isolated plasma reaction region is formed between the electrodes. A suitable etchant process gas is introduced into that region and, upon flow of rf energy in the region, a high energy wafer-etching plasma is produced which etches unmasked area of the wafer particularly silicon oxide layers on the wafer. A circular silicon-coated cap and diffusor is provided over the hot electrode, the diffuser being in the form of a flat circular disc or plate made from sintered aluminium cuttings and fitted within the cap interior. It has been found that the cap must be replaced after processing about 1,200 wafers as its 15 mils of silicon coating (to deter sputter induced metal ion contamination) is consumed during the etching process. A quartz focus ring surrounds the powered electrode cap to inhibit plasma coupled to other reactor surfaces. Further, with the diffusor employed, the cap has a propensity for arcing at points between the cap and diffusor. Arcing can cause burnt resist on the wafer and burn spots on the underside of the cap near the junction of the cap and diffusor as shown in the Prior Art FIG. 2.
Under rf current flow there will be a lag between the cap and the diffusor. The gap between the cap and diffusor (there are a finite number of points of contact between the cap and diffusor and consequently there is a gap everywhere else) forms the capacitor and the lateral conduction path through the cap forms the inductance in the equivalent Prior Art circuit shown in FIG. 6. A voltage difference between the cap and diffusor will (to the first order) be proportional to this lag (ac voltage difference). As the cap heats and expands in use, a point of contact between the cap and diffusor passing current comes out of contact and begins arcing. This arc generates ions and free electrons which can diffuse through holes in the cap. Once through the holes, they will act as seeds for carrier multiplication across the dark space. The resistance will be lower in this region which will have the effect of robbing power from neighboring regions and funnelling it into that one region; hence causing arcing. Once this occurs it will tend to be reinforced by the following phenomenon: The current in the plasma arc has an associated magnetic field which opposes lateral conduction along the cap to the diffusor. This will tend to increase the arcing between the cap and diffusor which, in turn, will increase the number of carriers generated under the cap.
To prevent contamination of the wafer on the opposite electrode, the Sigma 80 machine has the cap coated with a material (silicon) which will not be a contaminant when vaporized as it would not react with the plasma and would be pumped harmlessly out the exhaust. This, however, does consume reactive species and necessitates higher flow rates than would be needed were this consumption of the coating not taking place (higher process gas consumption). Also the silicon coating is consumed during etching, particularly rapidly on the outer ridge, resulting in a serious arcing problem and the need to replace the expensive cap when the silicon coating wears through.